Dockey Wood, Ashridge Estate. There was no charge when we went.
No good for public transport, though, I think?
Perivale Wood is open once a year on last Sunday in April for its show of bluebells. It's a short walk from Perivale Station on the Central line, about 45 minutes from Turnpike Lane.More details on this link
Problem is it's packed so no good if you want to commune with nature in solitude.
Dockey Wood is amazing but has a rather 'managed' feeling, with its carefully marked pathways. They are going to charge this weekend and next I believe. I understand why they've done this, but there are other parts of the Ashridge estate which feel more natural and are covered with bluebells this year as well - can't quite tell you where as it was part of a long walk we did.
As you say understandable, the consequence of letting people roam freely was the trampling much of the edges of bluebells, I think it looks better than last year, did you do the Old Copse Walk from the Bridgewater Monument?
We did a 7-mile circular from Tring station, so probably this was part of it - I was following a friend who knew the way so I don't have a mental picture of the route. Lovely walk!
Ightham Mote in Kent (National Trust) is the best bluebell carpet I've ever seen. It's in the wood next door so if not a member you can still go there.